Carboxylic acids hold promise as potential precursors for numerous chemicals. For example, succinic acid can serve as a feedstock for such plastic precursors as 1,4-butanediol (BDO), tetrahydrofuran, and gamma-butyrolactone. New products derived from succinic acid are under constant development, with the most notable of these being polyester which is made by linking succinic acid and BDO. Generally, esters of succinic acids have the potential of being “green” solvents that can supplant more harmful solvents and serve as precursors for millions of pounds of chemicals annually at a total market value of over $1 billion. Along with succinic acid, other 4-carbon dicarboxylic acids, such as malic acid, and fumaric acid also have feedstock potential.
Succinic acid can be used as a monomer for the production of various polyesters. It is commercially prepared by hydrogenation of maleic or fumaric acid, and is also produced by aqueous alkali or acid hydrolysis of succinonitrile. Currently more than 15,000 tons of succinic acid are manufactured annually in the United States.
The production of these carboxylic acids from renewable feedstocks (in this case through fermentation processes) is an avenue to supplant the more energy intensive methods of deriving such acids from nonrenewable sources. It has long been known that a mixture of acids are produced from E. coli fermentation. However, for each mole of glucose fermented, only 1.2 moles of formic acid, 0.1–0.2 moles of lactic acid, and 0.3–0.4 moles of succinic acid are produced. As such, efforts to produce carboxylic acids fermentatively have resulted in relatively large amounts of growth substrates, such as glucose, not being converted to desired product. Anaerobic production of succinate is hampered primarily by the limitations of NADH availability, slow cell growth and production.
Metabolic engineering has the potential to considerably improve process productivity by manipulating the throughput of metabolic pathways. Specifically, manipulating enzyme levels through the amplification, addition, or deletion of a particular pathway can result in high yields of a desired product.